So normally I don’t like nor participate in inter party combat but it’s very likely our Dm is ramping up the campaign to such a conflict. 

We have the HAlf elf dex redemption Paladin. Lv 5. Has a squire that’s his characters younger brother 

Human “half vistani” crossbow expert monster slayer lv 5 Not allowed to to use hand crossbow with sharpshooter/CBexpert. Given 20 silver arrows and a potion of night eye 

Human Assasin who’s a bit of a wild card lv 5. Has a poison he can use once which will do 3d6 first round, 2d6 second, etc. May switch sides for enough gold

Then those who didn’t change characters because they wanted to or didn’t have a choice

Halfling Divination Wizard with the eye of Vecna who has some severe evil tendencies due to said eye. A book that lets her cast any spell unprepared once per short rest, as well as she can use arcane recovery on a short rest. She can also upcast spells by 3 levels as well as use lv 6 Spells using the eye at a cost of having to do an evil action. (Dm favoritism?)  Lv 6

Dragonborn fighter battlemaster whose not terribly bright (as a character) nor morally grounded and follows the wizard. 20 Ac 18 strength. A breath weapon that levels. Also lv 6

Given the good possibility of an inter party fight I don’t see the monster slayer (myself), the Paladin and maybe the Assasin being able to defeat these two in the final battle. We’re all good friends and in game issues never have been a problem out of the game or else I’d never consider the possibility of this. Given all this, is it even feasibly possible to defeat the battlemaster and wizard?  

Uh... well if you don't want to fight the rest of the party, I'd recommend mentioning it to your DM and asking to avoid such a thing. If the table is cool with a battle royale... You're going to want the numbers advantage, so make sure you get the Rogue on your side. lol your Wizard is definitely way too overpowered, especially for Level 6... that would be rough to fight just as a DM controlled NPC. Considering your DM is giving one character an end-game magic item while not allowing one player to use a fairly common combination of feats makes me worry, though.

Not that I recommend it, but if your DM and IRL players are expecting an inter-party fight, the best option is to get surprise however you can and drop that wizard as soon as possible and/or take away all her casting focuses.  If possible, grapple her and hurl her into a pit of lava or off a cliff.  Assuming she didn't prepare spells like Feather Fall, Contingency, and that Eye doesn't have some kind of reaction spellcasting, this should kill her.

Quote from Gallazius >>

Halfling Divination Wizard with the eye of Vecna who has some severe evil tendencies due to said eye. A book that lets her cast any spell unprepared once per short rest, as well as she can use arcane recovery on a short rest. She can also upcast spells by 3 levels as well as use lv 6 Spells using the eye at a cost of having to do an evil action. (Dm favoritism?)  Lv 6 

 
Definitely DM favouritism going on there. Eye of Vecna is a Legendary item and your DM is giving the character abilities the Eye doesn't normally grant. Legendary items are typically Level 14+ at minimum. 

Neither the Halfling nor the Dragonborn have darkvision. Nor does the human assassin. So no matter who the assassin sides with, you'd best make your move under cover of darkness.

If the assassin sides with you, have them steal the Eye and the book while the Wizard is sleeping, and again attack by night.

And good luck.

Quote from JackOfAllTirades >>

Neither the Halfling nor the Dragonborn have darkvision. Nor does the human assassin. So no matter who the assassin sides with, you'd best make your move under cover of darkness.

If the assassin sides with you, have them steal the Eye and the book while the Wizard is sleeping, and again attack by night.

And good luck.

 Unless the DM is being even more generous with the Eye of Vecna than it's already clear they are, as written it's meant to be something where the character has to remove their own eye and replace it. If the DM is letting them just like... wear it as an amulet or something then you might as well not bother, because if that's the case the DM's favoritism will clearly kick in if you try anything.